Linear arrays of photovoltaic semiconductor diodes are used to produce higher voltages than a single diode can produce. Fabricating such arrays monolithically (on a common substrate) using mass production techniques common to the semiconductor industry is preferable to the manual assembly and interconnection of individual diodes. Monolithically fabricated arrays can be adversely affected by parasitic electrical currents flowing between the individual diodes through the substrate reducing the usable current and voltage produced by the array. One identified cause of parasitic substrate conduction is the generation of charge carriers in the substrate by illumination of the substrate in the regions between the diodes. Two methods have been identified to reduce or eliminate this effect; one is to use substrates that do not become conductive when illuminated and the other is to block the light from entering the substrate.
Using non-photo-conductive substrates has been demonstrated for silicon diode arrays but the silicon produced is either polycrystalline or amorphous. Silicon, being an indirect bandgap material, has an inherently low efficiency at converting photons into usable electrical current. Making the silicon polycrystalline or amorphous reduces the efficiency even further. The photon to-usable-current efficiency can be substantially improved by using single crystal, direct bandgap semiconductors. Single crystal, direct bandgap semiconductors are the preferred materials for high efficiency applications. However, fabricating high quality, single crystal semiconductor diodes on insulating substrates has been very difficult. Substrates which allow the formation of single crystal diodes tend also to be susceptible to photo-generated carrier production.
Methods for blocking the light from entering the substrates required for the fabrication of single crystal, direct bandgap semiconductors have also been demonstrated. Such light blocking techniques have been limited to small differences in electrical potential (10 volts or less) between spaced diodes. The reduction of current leakage between closely spaced diodes in photodetector diode arrays has been discussed in U.S. Pat. Nos. 5,049,962; 5,061,652; and 6,133,615. Other approaches have been based on doped direct bandgap materials (e.g., U.S. Pat. No. 4,774,554 to Dentai, et al. An example of a photodiode array containing direct bandgap buffer materials on insulating substrates is discussed by Major et al (U.S. Pat. No. 6,100,546). However, a need still exists for photodiode arrays for high voltage power generation, using direct bandgap materials on semi-insulating substrates.
Briefly, the invention includes an array of photodiodes on a semi-insulating substrate, a method for fabricating such photodiodes, and a system for converting optical energy to high voltage electrical energy. The reduction or elimination of the injection of electrical charge carriers into a common semi-insulating substrate having an array of two or more photodiodes (e.g., photocells) at different electrical potentials is provided by positioning a non-conductive buffer layer containing at least one single crystal semiconductor material between the substrate and spaced diodes. Such an array can provide reduced current injections and voltage collapse and can be illuminated with uniform light beam profiles to increase efficiency.
The invention provides high quality lattice matching (i.e., within about 5%) between the semiconductor materials of the non-conductive single crystal, buffer layer and the semiconductor materials of the mesa photodiodes on the array, particularly during epitaxial growth fabrication of the array.